Electro-chemically machined ring and strut structure for prosthetic heart valves

ABSTRACT

This invention relates to the electrochemical machining of integrally formed prosthetic heart valve structures in which both the structural integrity of the valve and the blood flow through the valve is improved as a result of the integral formation of the valve structure. In particular, such heart valves comprise a ring and a pendant structure, such as a strut, for supporting the opening and closing of a discoid valve occluder. Integral formation of the strut with the ring permits the cross-sectional shape of the strut to be machined without overheating and distortion to conform to a teardrop shape having favorable fluid-dynamic characteristics thereby improving the flow of blood through the valve across the strut. The integral formation of the strut with the ring further results in the elimination of the weld fillet between the strut and the ring, thereby permitting the height of the ring to be reduced, which further improves the fluid dynamic characteristics of the valve. The overall structural integrity of the heart valve structure is also improved by the shape of the strut which permits the increase in the total cross-sectional area of the strut while retaining the favorable fluid flow characteristics of the valve. Thus, this invention provides for improved structural integrity of the valve and at the same time, improved flow characteristics through the valve.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-part of copending application Ser.No. 870,685, filed Jan. 19, 1978 and now abandoned.

BACKGROUND OF THE DISCLOSURE

Since the early 1960s, artificial heart valve prostheses have enjoyedtremendous success. Reliability and sustained high performance has beenachieved by the use of very hard and exotic materials to form thestructure. Such heart valve prostheses are exemplified in the followingU.S. Pat. Nos. 3,534,411-Shiley; 3,812,542-Shiley; 3,824,629-Shiley;4,057,857-Fettel, and the disclosures of these patents are incorporatedherein by reference. Typically, such heart valve prostheses have a ringstructure. Attached to the ring structure is usually some form of strutor pendant structure of elongated shape which supports the opening andclosing of a discoid occluder. This occluder is disposed over the ringstructure and substantially seals the orifice defined by the ringstructure. In the art of heart valve prostheses, a constant search forimprovement of the valve has continued since the early 1960s. The goalsfor such a search have been the reduction of thrombus formation in theheart within the vicinity of the heart valve through the reduction ofstasis in blood flow (which stasis is roughly proportional to the sizeof the obstruction to blood flow presented by the heart), and concurrentwith this goal, the goal of increasing the structural integrity of theheart valve. However, it has seemed that these two goals wereincompatible in that increasing the structural integrity of the valveinvolved increasing the size of the structural members associated withthe valve. Such an increase in size presents a greater obstructionpresented to the blood flow with a concurrent increase in stasis, whichthus increases the potential for thrombus formation. Thus, the searchhas continued for a method for improvement which would reconcile the twoseemingly competing and irreconcilable goals of structural improvementand blood flow improvement.

Typically, in the prior art, the pendant structures or struts, attachedto the valve ring, are formed of very hard metal wire, such as Haynes 25material or a cobalt alloy, which is cut and welded to the ringstructure after being shaped to the appropriate shape. Such weldingnecessarily results in the formation of a weld fillet extending beyondthe welded juncture between the strut and the ring which has to beaccommodated on the surface of the valve ring. Furthermore, these wirestruts have a circular cross-section over which the blood has to flow.Although a circular cross-section for an obstruction sitting across theblood flow is not the most favorable cross-section for maximizing theflow across the valve, attempts to modify the circular cross-sectionalconfiguration of the wire strut were generally unsuccessful because themachining of such a hard metal elongated wire strut leads to overheatingof the wire and consequent distortion of the wire. These effects are dueto the hard metal from which the wire must be formed and the prolongedmachining time required to shape such a wire.

The prior art generally taught against integral formation of the strutand the valve ring in heart valves. The use of very hard metallicalloys, such as Haynes 25 material, or various alloys of cobalt, madethe machining of such metal extremely difficult. The intricate shape ofthe strut necessary to permit the wobbly tilting action disclosed in theFettel patent, U.S. Pat. No. 4,057,857, and the unique rocking occluderaction also disclosed in that patent, made the machining of the struteven more difficult and seemingly impossible. The cross-sectional areaof the strut is thin compared to its length, and this was a furtherobstruction in the machining of such a strut. It was found, for example,when the integral formation of the strut and ring was attempted throughnumerical control machining of a disc-shaped blank, the small size andintricate shape of the strut caused overheating and distortion in thestrut because of the low heat conductive characteristics of the thinstrut and because of the lengthy machining time necessary to achieve theintricate shape required for the strut. In stamping, it was found thatthe edge finish of the strut was not acceptable and the teardrop shapecould not be sheared properly. When forging was attempted to form theintegral strut and ring structure from a disc blank, the surface finishwas bad as a result of the forging process and the costs in machiningthe finish after forging were prohibitive. Investment casting was alsoattempted in order to form the strut integral with the valve ring.However, such casting resulted in a porous surface finish which againresulted in the requirement for additional costly machining in order toobtain a good surface finish. As is well known in the art, a smoothsurface finish is necessary in order for the heart valve prosthesis towork successfully. Also, the formation of voids within the structure bycasting presented a serious risk. Laser machining was also attempted toform the strut and ring integral with one another from a disc blank.However, the teardrop shape of the strut could not be formed with lasermachining and the inside ring radius could not be formed. Because of thetight dimensional tolerances inherent in the heart valve design,electrical discharge machining was found to be impractical becausedimensional control in such machining is difficult due to erosion ofmetal from the electrode of the tool. Furthermore, the outer peripheryof the ring could not be formed using electrical discharge machining.Finally, in the effort to form the strut and the valve ring to beintegral with one another, chemical milling was attempted. However,because of the slow etch rate of the hard material from which the valvering and strut must be formed, the mask used in the chemical milling wasunavoidably undercut in the attempt, and the shape of the resultingstructure was distorted. Thus, it seemed apparent in the prior art thatintegral formation of the strut and ring was not a viable or practicalsolution to the problem of improving the structural integrity of thevalve structure and improving the blood flow across the valve.Increasing the size of the cross-sectional area of the wire, whileleading increased structural integrity to the juncture between the wirestrut and the valve ring, unavoidably increases the pressure gradientacross the valve by offering a larger obstruction to the blood flowthrough the orifice defined by the ring.

As an alternative method, the machining of a disc-shape blank to form avalve ring integral with a strut in a single integral structure has metwith similar difficulties, discussed below. Furthermore, the advantagesto be gained by integral formation of the strut and ring have not beenfully appreciated in the teachings of the prior art.

SUMMARY OF THE INVENTION

In this invention the integral formation of the valve ring and discoidsupporting strut is taught. Such integral formation is made possible bythe process of electrochemical machining. Such integral formation of thering and the strut enables the strut to be configured in teardropcross-section which substantially improves the fluid-dynamiccharacteristics of the valve, thereby improving blood flow through thevalve, since this shape offers substantially less fluid-dynamicresistance to blood flowing around the strut. This, in turn, permits thecross-sectional area of the strut to be increased without detractingfrom the favorable fluid-dynamic characteristics of the valve. Theincrease in cross-sectional area of the strut, in turn, increases thestrength of the juncture between the strut and the ring, sinceincreasing the cross-sectional area of the strut increases the amount ofmetal supporting the juncture between the strut and the ring. Thus, thegoal of improving structural integrity of the valve and the goal ofimproving th blood flow through the valve in order to reduce thrombusbuildup have both been achieved by the implementation of this invention.

In the incorporation of this invention as an improvement in the pivoteddiscoid heart valves as taught and claimed in U.S. Pat. Nos. 3,824,629and 4,057,857, the closing support strut is formed integrally with thering and given a teardrop cross-section. The closing support strut ischosen as opposed to the opening support strut in this type of valvebecause the closing support strut is larger and is more important in itseffects on blood flow and structural integrity of the valve. However, ifdesired, the opening support strut can also be formed in the same manneras the closing support strut.

In the incorporation of this invention as an improvement in the centerpost heart valve, as taught and claimed in U.S. Pat. No. 3,812,542, thecantilevered support strut of the center post which lies across the flowfield of the blood is chosen to be formed integrally with the valve ringand to be given the teardrop shape. Similarly, in the heart valve havinga pair of parallel cage struts which are symmetrically disposed to oneanother, as taught and claimed in U.S. Pat. No. 3,534,411, the verticalportions of these struts which lie across the blood flow opening of thevalve are chosen to be formed integrally with the valve ring and aregiven the teardrop-shaped cross-sectional configuration.

With respect to the pivoted discoid heart valves, the use of thisinvention as an improvement results in a two-fold improvement of thecharacteristics of the valve. As in all applications of this inventionto the typical heart valve, the strength of the juncture between thestrut and the valve ring is improved due to the increasedcross-sectional area of the strut, and the fluid flow around the strutis improved by the introduction of the teardrop cross-sectional shape ofthe strut. In addition to these improvements, the required height of thering is further reduced because the weld fillet, which necessarilyextends beyond the juncture between the strut and the ring, iseliminated by the integral formation of the strut and the ring. Thus,the height of the ring need not be great enough to accommodate theextending weld fillet, and may be reduced by the width of the weldfillet which has been eliminated, thereby locating the valve ring edgeflush with the junction between the strut and the valve ring. Thisreduced height of the valve ring reduces the amount of obstructionpresented to the blood flow across the orifice defined by the ring,thereby improving flow characteristices and reducing stasis in the bloodflow and formation of thrombus in the vicinity of the ring. Conversely,because the minimum height requirement of the ring has been reduced, thecross-sectional area of the strut may be increased without necessarilyincreasing the height of the ring.

In summary, with respect to pivoted discoid heart valves, this inventionpermits the strengthening of the juncture between the closing supportstrut and the valve ring and the improvement of the blood flowcharacteristics through the orifice of the valve by integral formationof the closing support strut with the valve ring. This integralformation permits the machining of the strut to a teardrop shape andeliminates the weld fillet at the juncture between the strut and thevalve ring. Thus, increasing the cross-sectional area of the strut doesnot present a greater obstruction to the blood flow in the vicinity ofthe strut because the introduction of the teardrop shape reduces thefluid dynamic resistance presented by the strut. Also, the increase inthe size of the cross-sectional area of the strut does not present anincreased obstruction to blood flow in the vicinity of the ring becausethe elimination of the weld fillet reduces the minimum heightrequirement of the ring. In fact, by the introduction of the teardropcross-sectional shape of the strut, and by the minimization of therequired height of the ring through the elimination of the weld fillet,the blood flow through the orifice of the valve is substantiallyimproved.

The integral formation of the strut with the ring has been made possiblethrough this invention by the electrochemical machining of the valve.Electrochemical machining involves holding the workpiece, such as thedisc-shaped blank, from which the heart valve is to be formed, close toa negative electrode or cathode, specifically within about 0.010 inches.The workpiece is charged positively and an electrolytic solution isforced, under high pressure, approximately 175 psi, in between theworkpiece and the negative electrode. A high current density, ofapproximately 1000 amperes per square inch of machined area, is passedthrough the electrolytic solution between the negative electrode and thepositively charged workpiece. Metallic ions are removed from theworkpiece and placed into the electrolyte solution and thereby kept fromforming on the cathode and distorting the cathode. The machining israpid due to the high current density between the workpiece and thecathode. As applied to the heart valve, and in particular as applied tothe pivoted discoid heart valve or the heart valve with arcuateoccluder, the cathode takes the form of a die from which an enlargedheart valve structural could nearly be cast. The workpiece initially isa disc-shaped blank, the diameter of which is slightly larger than theoutside diameter of the heart valve ring and the thickness of which isslightly greater than the height of the ring of the heart valve. Whilein the electrolytic solution and during the electrochemical machining,the cathode is fed down onto the workpiece, and a valve ring and closingsupport strut are formed by the removal of metal from the blank due tothe current flow of metal ions from the blank into the electrolyticsolution. The teardrop shape and the intricate structure of the closingsupport strut are successfully formed by the electrochemical machiningprocess. The overheating or distortion of the strut prevalent in theprior art is eliminated through electrochemical machining, and thedimensional tolerances are tightly held in this machining process.

For a more thorough understanding of this invention, reference may behad to the following detailed description and drawings wherein:

FIG. 1 is a diagramatic sectional view of a heart with the prosthesis ofthis invention inserted in place of the natural mitral and aortic heartvalves;

FIG. 2 is a top perspective view of the pivoted discoid heart valveprosthesis constructed in accordance with this invention;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2;

FIG. 4 is a diagramatic cross-sectional viewof the heart valveprosthesis of FIG. 3 taken substantially along curved line 4--4 of FIG.2;

FIG. 5 is a cross-sectional view of the heart valve prosthesis takenalong line 5--5 of FIG. 2 but showing the valve occluder in an openedposition;

FIG. 6 is a top plan view of a pivoted discoid heart valve prosthesishaving an arcuate occluder constructed in accordance with thisinvention;

FIG. 7 is a cross-sectional view of the prosthesis of FIG. 6 taken alongline 7--7 of FIG. 6.

FIG. 8 is a cross-sectional view of the prosthesis of FIG. 6 taken alongline 8--8 of FIG. 6;

FIG. 9 is an enlarged cross-sectional view of the middle portion of theclosing support strut of FIG. 8 taken along line 9--9 of FIG. 8;

FIG. 10 is an enlarged cross-sectional view of the outer portion of theclosing support strut of FIG. 8 taken along line 10--10 of FIG. 8;

FIG. 11 is a cross-sectional view of the prosthesis similar to FIG. 7but showing the valve occluder in an open position;

FIG. 12 is a top perspective view of a heart valve prosthesis withparallel cage struts constructed in accordance with this invention;

FIG. 13 is an exploded view of the heart valve prosthesis of FIG. 12;

FIG. 14 is a cross-sectional view of one of the parallel cage struts ofFIG. 13, taken along line 14--14 of FIG. 13;

FIG. 15 is a top perspective view of a center post heart valveprosthesis constructed in accordance with this invention;

FIG. 16 is a cross-sectional view of the heart valve prosthesis of FIG.15 taken along line 16--16 of FIG. 15;

FIG. 17 is a view similar to FIG. 16 but showing the valve occluder inthe open position;

FIG. 18 is a cross-sectional view of the attachment leg of FIG. 17,taken along lines 18--18 of FIG. 17;

FIG. 19 is a cutaway cross-sectional view of the electrochemical machinetooling assembly used to form the heart valve with arcuate occluderillustrated in FIGS. 6, 7, 8, 9, 10, and 11;

FIG. 19A is a cross-sectional view taken along line A--A of FIG. 19;

FIG. 20 is a detailed bottom perspective view of the electrode used toform the proximal side of the valve structure;

FIG. 21 is a bottom plan view of the electrode of FIG. 20;

FIG. 22 is a cross-sectional view of the electrode of FIG. 21 takenalong line 22--22 of FIG. 21 showing the electrode fitting over theproximal side of the valve structure being formed;

FIG. 23 is a cross-sectional view of the electrode of FIG. 21 takenalong line 23--23 of FIG. 21;

FIG. 24 is a top perspective view of a blank used to form the valvestructure;

FIG. 25 is a top perspective view of the completed proximal side of thevalve structure after the electrochemical process using the electrode ofFIGS. 20 through 23 has been completed;

FIG. 26 is a bottom perspective view of the structure of FIG. 25;

FIG. 27 is a bottom perspective view of the electrode used to form thedistal side of the valve structure;

FIG. 28 is a bottom plan view of the electrode of FIG. 27;

FIG. 29 is a cross-sectional view of the electrode of FIG. 28 takenalong line 29--29 of FIG. 28 showing the electrode fitting over thedistal side of the valve structure being formed; and

FIG. 30 is a top perspective view of the completed valve ring and strutstructure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is illustrated of the general concept of heart valve prosthesisin which an artificial heart valve is implanted in place of a removednatural heart valve. The aortic and mitral valve prostheses areillustrated at 10 and 10' respectively. The aortic artery is illustratedat 12. The natural valve ring is illustrated at 14. The artificial heartvalve 10 sits on the natural valve ring for an aortic heart valveprosthesis, for example. For the mitral heart valve prosthesis, theartificial heart valve 10' sits on the natural valve ring muscle tissueindicated at 18 at the left atrium 20. The distal side of the valve isdefined to be the outlet or downstream side of the valve, while theproximal side of the valve is defined to be the inlet or upstream sideof the valve. As is apparent from FIG. 1, the size of the valve variesdepending upon the type of prosthesis for which it is designed.

DESCRIPTION OF THE FIRST EMBODIMENT

In the first embodiment of this invention, the closing support strut ofthe pivoted discoid heart valve, as disclosed and claimed in U.S. Pat.No. 3,824,629-Shiley, is formed to be integral with the ring structureof that valve. From this description, it will become apparent that thefull advantages for which this invention was intended are realized,namely that the flow of blood is improved because of favorablefluid-dynamic characteristics of the valve, the size of the valve issubstantially reduced, and the structural integrity of the valvestructure is improved. FIG. 2 shows a top perspective view of thepivoted discoid heart valve corresponding to the aortic heart valveprosthesis indicated in FIG. 1 at 10. The circular valve ring structureor body is indicated at 22 and it defines the orifice through which theblood must flow to enter the aortic artery. The valve occluder 24 isshown in FIG. 2 in the closed position. This view shows the distal sideof the valve occluder 24. The valve occluder 24 is generally in a discor discoid shape. The term discoid is used here to denote any geometricshape which is basically cylindrical in shape and in which the heighthof the cylinder is small in comparison with the diameter of thecylinder. The distal surface of the valve occluder 24 is divided intotwo regions; namely, an outer annular region, designated at 38 in FIG.2, and a depressed, circular inner area, designated at 44. This innerdespression fits the opening support strut 26 within the depressed area44. The opening support strut 26 is comprised of two diverging strutelements 58, symmetrically disposed as a pair and attached to the valvering 22. Integral with the pair of diverging strut elements 58 are apair of shoulders 61 curving inwardly toward one another and merginginto a base 60 which contacts the wall 64 of the depressed area 44. Theclosing support strut 28 is indicated by dashed lines as being disposedon the proximal side of the valve occluder 24.

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2 inwhich the closing support strut 28 is fully illustrated along with theshape of the discoid occluder 24. The relative thickness of the outerannular portion 38 and the relative thinness of the depressed inner area44 of the discoid occluder 24 are apparent in FIG. 3. The base or middleportion 60 of the opening support strut is illustrated as impinging onthe distal surface of the depressed area 44 of the occluder 24. Theclosing support strut 28 includes a pair of strut elements 62 formingthe two junctures with the valve ring diverging inwardly of the ring 22into a pair or proximally extending smooth curved shoulders 66. Thesesmooth curved shoulders 66 merge into a middle portion 64 of the closingsupport strut 28, and the middle portion 64 curves proximally away fromthe valve ring 22.

FIG. 4 shows the distal surface 36 of the valve occluder 24 and theproximal surface 40 which sits on the proximally extending curvedshoulder 66 of the closing support strut 62. The action of the rotationof the valve occluder 24 is indicated in the phantom outline of theoccluder in FIG. 4 in which the pivot point is moved due to the rotationof the valve occluder 24.

FIG. 5 illustrates the heart valve of FIG. 3 in which the vane ordiscoid occluder 24 is in the opened position.

A significant feature of this invention is that the integral formationof the closing support strut 28 with valve ring 22 eliminates the weldfillet between the valve ring and the closing support strut. As aresult, the lower or proximal edge 70 of the valve ring 22 is enabled tobe flush with the lower edge or proximal edge 72 of the valve ring 22,as illustrated in FIGS. 3 and 5. Making reference now to U.S. Pat. No.3,824,629, and in particular, FIGS. 5 and 6 of that patent, it may beseen that in the prior art the ring extended below the proximal edge ofthe closing support strut. This extra extension of the ring wasnecessary in order to accommodate the weld fillet which extended beyondthe juncture between the ring and the closing support strut. With theintegral formation of the closing support strut and the valve ring asaccomplished by the present invention, the weld fillet has beeneliminated and along with it the requirement for the extension of thering beyond the lower edge 72 of the closing support strut 28. In thisway, the cross-sectional area of the closing support strut 28 may beincreased without increasing the height of the ring 22.

Reducing the height of the ring 22 reduces the amount of obstructionpresented to the blood flowing across the valve thereby increasing thefavorable fluid dynamic characteristics of the valve. Also, increasingthe cross-sectional area of the closing support strut 28 increases thestrength of the juncture between the closing support strut 28 and thering 22.

The integral formation of the closing support strut 28 with the valvering 22 is achieved by electrochemical machining a discoid blank pieceof metal, as described more fully below. This eliminates the necessityof using a wire to form the closing support strut 28, as exemplified inthe prior art, U.S. Pat. No. 3,824,629. Therefore, the cross-sectionalshape of the closing support strut 28 need not be circular but may beany shape which may be machined electrochemically. In particular, theshape illustrated in FIG. 5 for the cross-section 74 of the closingsupport strut 28 is shown to be a modified teardrop shape. This teardropshape has favorable fluid-dynamic flow characteristics such that theresistance to blood flow around the strut is substantially reducedwhile, at the same time, the total cross-sectional area of the closingsupport strut may be increased. As discussed before, the prior arttaught that increasing the thickness of the circular strut wouldinherently offer greater hydrodynamic resistance to the hemodynamic flowof the blood. However, with the teardrop cross-sectional shape of theclosing support strut 28, the resistance to flow is actually decreased,even though the cross-sectional area of the closing support strut may beincreased. Because the cross-sectional areas of both the closing supportstrut and the juncture 76 have been increased, the amount of metal whichforms the juncture 76 between the valve ring 22 and the closing supportstrut 28 is substantially increased, thereby correspondingly increasingthe structural integrity of the structure of the valve ring to asubstantial degree. In addition, the flow characteristics of the valveare substantially improved. The incorporation of this invention as animprovement into the pivoted discoid heart valve reconciles the twocompeting goals long sought after in the art, namely, the goal ofincreasing the structural integrity of the structure, and the goal ofimproving the fluid dynamic characteristics of the valve so as todecreases tasis and thrombus formation in the vicinity of the valve.

Description of the Second Embodiment

FIG. 6 is a top plan view of a heart valve having an arcuate occluder astaught and claimed in U.S. Pat. No. 4,057,857, constructed in accordancewith this invention, and showing a structure very similar to that of thepivoted discoid heart valve, previously discussed. The opening supportstrut is fully visible in FIG. 6 and designated generally at 26a. Thisstrut consists of two inwardly disposed struts 58a forming a pair ofrounded shoulders 61a with a middle portion 60a. The middle portion 60aabuts the wall of a central depression 44a in an arcuate occlunder 24a.The closing support strut is illustrated by the hidden lines anddesignated generally at 28a. The closing support strut 28a is disposedon the proximal side of the arcuate occluder 24a. The strut 28a iscomprised of two inwardly extending support struts 62a merging into apair of curved shoulders 63a passing through the axial center line 65aof the valve and formed through a pair of right-angles forming shoulders66a which are formed on either side of a middle portion 64a.

The closing support strut 28a is formed integrally with a valve ring22a. Therefore, the cross-sectional shape of the closing support strut28a need not be circular. The closing support strut 28a and the valvering 22a are preferably formed from a disc-shaped blank andelectrochemically machined, as discussed below. Thus, the shape of thecross-sectional configuration of the closing support strut 28a may beselected from any shape which is fluid-dynamically favorable to flowthrough the orifice defined by the ring 22a. A modified teardropcross-sectional shape 74a is illustrated as exemplary in FIG. 7, and isparticularly suited to decreasing the hydrodynamic resistance to theflow around the closing support strut 28a.

FIGS. 9 and 11 show the cross-sectional shape 74a of the middle portion64a of the closing support strut 28a. This shape illustrated is amodified teardrop shape designed to facilitate the valve closing action.Thus, the modified teardrop shape indicated at 74a is chosen toimplement the rocking action discussed above and disclosed in U.S. Pat.Nos. 3,824,069 and 4,057,857 in which the pivot axis of the occluderchanges as the occluder rotates between the open and closed position.Thus, the modified teardrop shape of the cross-sectional configurationof the middle portion 64 is chosen to be compatible with the feature ofthe changing pivot axis and the wobbly tilting action disclosed in U.S.Pat. No. 4,057,857. The modified teardrop shape indicated at 74a of themiddle portion 64a of the closing support strut is further preferablychosen to permit the occluder to rotate to the open position defined bythe angle a in FIG. 11.

The portion 62a of the closing support strut 28a have an equilateralteardrop shape cross-section as illustrated in FIG. 10. FIGS. 9 and 10thus illustrate the complete flexibility in the choice ofcross-sectional shapes made possible by the integral formation of theclosing support strut with the valve ring taught by this invention. Asillustrated, the cross-sectional shape need not be uniform across theentire closing support strut, but may vary across the length of theclosing support strut for the purpose of maximizing fluid flow throughthe valve ring and minimizing the pressure gradient across the valve.

The integral formation of the valve ring and the closing support strutalso eliminates the need for a weld fillet at the juncture between theclosing support strut and the valve ring. Thus, as shown in FIGS. 7, 8and 11, the proximal or lower edge surface 70a of the valve ring and theproximal or lower edge surface 72a of the closing support strut 28a maybe formed to be flush with one another.

The lower or proximal edge 70a of the ring and the lower proximal edge72a of the closing support strut may be formed flush with one anotherbecause the weld fillet used in the prior art at the juncture betweenthe valve ring and the closing support strut has been eliminated due tothe integral formation of the ring and the closing support strut. In theprior art, it was necessary, in forming the heart valve with arcuateoccluder, that the lower or proximal edge 70a of the valve ring extendbelow the lower or proximal edge 72a of the closing support strut 28a inorder to accommodate the weld fillet which necessarily extended belowthe proximal edge 72a of the closing support strut 28a. With theelimination of this weld fillet, the thickness of the closing supportstrut may be increased and the height of the ring may be increased tosuch a point that the lower or proximal edge 70a of the ring and thelower or proximal edge 72a of the closing support strut are flush withone another and the ring does not extend below, or proximally of, thejuncture between the valve ring and the closing support strut. Thiseliminates the amount of ring structure which is presented to the bloodflow, and thereby improves the blood flow by decreasing the amount ofobstruction presented thereto across the orifice defined by the valvering 22a. Further, the aforementioned increase in the thickness of theclosing support strut increases the structural integrity of the juncturebetween the closing support strut and the valve ring. This is due to thefact that an increase in the cross-sectional area of the closing supportstrut increases the amount of metal involved in forming the juncturebetween the valve ring and the closing support strut.

Again, the two competing goals long sought after in the art, namely,increasing the structural integrity of the valve and the goal ofimproving the fluid flow through the valve, have both been reconciledand fulfilled by the implementation of this invention as an improvementin the heart valve with arcuate occluder. The structural integrity hasbeen increased because the thickness of the closing support strut hasbeen increased, thereby strengthening the juncture between that strutand the valve ring. The fluid flow characteristics through the valvehave been increased because the minimum height requirement of the valvehas been decreased by the elimination of the weld fillet, and by themodified teardrop-shaped cross-sectional configuration of the closingsupport strut. All this has been made possible by the formation of thevalve ring and closing support strut as an integral structure formedfrom a disc-shaped blank by the process of electrochemical machining.

Description of the Third Embodiment

Turning now to FIG. 12, a top perspective view of a parallel cage strutheart valve prosthesis is illustrated, corresponding to the aortic valveprosthesis illustrated in FIG. 1 at 10. FIG. 13 is an exploded view ofthis heart valve prosthesis and shows a ring 102 defining an orifice 104which is sealed by a discoid occluder 108 which sits over the ring 102.As is apparent from FIG. 12, the movement of the discoid occluder 108 isrestricted and bounded by a pair of parallel cage struts 114 which aresupported on an annular retaining ring 106. The parallel cage struts 114each comprise a pair of support legs 144 which extend vertically andperpendicularly from the annular retaining ring 106 and are joinedtogether by a horizontal stop portion 142 which extends perpendicularlyof the legs 144 and in general parallel relation to the plane defined bythe outer perimeter of the circular annular retaining ring 106. The ring102 is held within the inner annulus defined by the annular retainingring 106 by means of an upper flange 118 and a lower flange 120 integralwith the ring 102. For this purpose, a portion of the annulus of theannular retaining ring 106 fits inside an annular groove 122 between theupper flange 118 and lower flange 120 of the ring 102, thereby firmlyholding the ring 102 within the annular retaining ring 106. The discoidoccluder 108, when in the closed position, sits over the upper surface124 of the ring 102. The discoid occluder 108 travels perpendicular tothe plane defined by the outer circle of the annular retaining ring 106,and abuts the stop portion 142 of each of the support struts 114 in itsfull open position. After passing through the orifice defined by thering 102, the blood must flow radially of the circular annular retainingring 106 and around each of the legs 144. For this reason, thecross-sectional shape of the legs 144 determines the amount offluid-dynamic resistance to blood flow through the valve and thereforedetermines the pressure gradient across the valve.

The use of an annular cloth pad 146 and an annular cloth sleeve 148 withthe valve of FIG. 12 to provide aortic heart valve prosthesis 10illustrated in FIG. 1 is disclosed and claimed in U.S. Pat. No.3,534,411.

In the valve of FIG. 12, the pair of parallel cage struts 114 are formedintegrally with the annular retaining ring 106. During the machining ofthe disc blank from which the annular retaining ring 106 and theintegral parallel cage struts 114 is machined, a teardropcross-sectional shape is advantageously formed in each parallel cagestrut. FIG. 14 illustrates the disposition of such a teardropcross-section in each of the legs 144 which substantially improves thefluid flow characteristics through the valve. An acutely arcuate vertex145 of the teardrop shape is presented to the blood flow and, therefore,presents less fluid-dynamic resistance to the flow. This lowers thepressure gradient across the valve and improves the blood flow andwashing action of the blood to minimize thrombus formation in thevicinity of the valve. Thus, due to the use of the teardropcross-section in the leg 144, the cross-sectional area of the struts maybe enlarged and thickened without impeding the flow of blood through thevalve.

In summary, the improvement incorporated into the parallel cage strutheart valve consists in forming each of the parallel cage struts 114integrally with the annular retaining ring 106, and integrally formingthe parallel cage strut legs 144 to have a teardrop shape in which theacute vertex of the teardrop points opposite to the direction of theblood flow such that the vertex is presented to the blood flowing pastthe leg 144. The machining of the shape of the parallel cage strut,heretofore impossible because of heat loss and distortion due to therelatively long length and small thickness of the cage strut 114, ismade possible by the electrochemical machining of the shape of the strutfrom a large disc-shaped blank from which the strut and the annular ring106 are integrally formed.

Description of the Fourth Embodiment

FIGS. 15-18 illustrate this invention incorporated in a center postheart valve. This valve is the subject of U.S. Pat. No. 3,812,542. Thecenter post heart valve may be used in the prosthesis as illustrated inFIG. 1 and indicated at 10. The valve is comprised of an annularretaining ring 216 which has a distal side 224 and a proximal side 222.The retaining ring 216 defines a blood flow orifice 214 through thecenter thereof. A frusto-conical discoid occluder 240 sits on the distalside 224 of the annular retaining ring 216 forming a seal for theorifice 214 on the distal side 224 of the ring 216. The movement of theoccluder 240 is restrained by the L-shaped guide member or strut 228which passes through a tight-fitting opening 242 in the occluder 240.The L-shaped guide member is comprised of a first leg 230 extendingtransversely from the retaining ring 216. A shoulder is formed at 234 inwhich the leg is merged into a center post portion 323 of the L-shapedstrut 228. The opening 242 in the occluder 240 fits around the centerpost 232 in such a manner as to accurately guide the movement of theoccluder up and down along the valve structure between the open and theclosed position illustrated respectively in FIG. 17 and FIG. 16. At thedistal end 236 of the center post 232 a stop knob 238 is disposed firmlyand symmetrically with respect to the center axis 235 of the center post232. When the occluder 240 is in the full opened position, asillustrated in FIG. 17, the bottom 254 and the perimeter 252 of the stopknob 238 are seated in a reces 248 in the distal surface 246 of theoccluder 240 such that the stop knob perimeter 252 abuts against theperimeter 250 of the recess 248. The depth 256 of the recess 248 issufficient to assure fluid damping of the stopping motion between thebottom surface 254 and the recess 248. When the occluder is in theclosed position illustrated in FIG. 16, the proximal face 244 of theoccluder 240 rests on distal surface 244 of the annular ring 216 toprovide a seal which closes the valve.

In use as heart prosthesis, blood flows through the orifice 214 in adirection away from the proximal side 222 of the ring and towards thedistal side 224 of the ring. From this it is apparent that thetransverse leg portion 230 of the L-shaped guide member 228 acts as anobstruction around which the blood must flow. This obstruction ispreferably minimized. In the prior art, exemplified in U.S. Pat. No.3,812,542, the minimization of the obstruction presented by the leg 230to the blood flow through the orifice is achieved by the minimization ofthe cross-sectional area of the leg 230. As taught by U.S. Pat. No.3,812,542, the L-shaped member 228 is preferably formed by a singlepiece of wire having a constant diameter of approximately 1/16th of aninch. Because of the cantilever fashion of the attachment of the strut228 to the ring, the juncture between the leg 230 and the ring 240 iscritical for the structural integrity of the valve. Such structuralintegrity would be improved by any increase in the thickness of the wireat the leg portion 230. However, such an increase in thickness wouldcorrespondingly increase the amount of obstruction presented to theblood flowing through the orifice 214, and thereby increase the pressuregradient across the valve.

Contrary to the teaching of the prior art, in this invention the centerpost 232 and the cantilevered leg 230 are formed integrally with ring214 from a single disc-shaped blank using the process of electrochemicalmachining. This permits a fine machining to be carried out on the shapeof the cantilever support member. 228 to give it a teardropcross-sectional configuration at the leg portion 230 which is disposedacross the blood flow to the orifice. This cross-sectional shape is bestillustrated in FIG. 18, which illustrates the preferred orientation ofthe teardrop cross-section of the strut 230 wherein an acutely arcuatevertex 245 of the teardrop is pointed into the oncoming blood flow.

According to the prior art, because the cantilevered support member 228had been formed of wire, preferably welded to the annular support ring214, any machining or changing in shape of the cross-sectionalconfiguration of the strut would have resulted in overheating anddistortion due to the long machining time required because of thehardness of the materials used to form the valve. According to thisinvention, the teardrop shape cross-section of the strut 230,illustrated in FIG. 18, decreases fluid-dynamic resistance to blood flowthrough the valve and thereby permits an increase in the cross-sectionalarea of the leg 230. This increase in cross-sectional area results in astronger juncture between the ring 216 and the leg 230. This increase instructural integrity is due to the additional amount of metal within thejuncture between the ring 216 and leg 230.

Electrochemical Machine Tooling Assembly

FIGS. 19 and 19A illustrate the electrochemical machine tooling assemblyused in the electrochemical machining of an integrally formed valve ringand closing support strut, and particularly the strut 28a illustrated inFIGS. 6-11.

Referring first to FIG. 19, an electrode support 302 is moveably mountedfor vertical translation on two alignment rods 304. The rods 304 aresupported in a locating stationary base 306. An electrode mounting head308 is suspended above the base 306 by the support 302. As shown in FIG.19, a proximal side forming electrode or die 312 (which is more clearlyillustrated in FIGS. 20-23, and which will be described in detailhereinafter) has an annular mounting flange 314 which seats against theunderside of the mounting head 308. The electrode or die 312 isremovably attached to the mounting head 308 by means of a plurality ofbolts 316 (as illustrated in FIG. 19A) which pass through the mountingflange 314 into the head 308.

A sleeve 318 is mounted in a slideable engagement around the mountinghead 308 by means of a set screw 320 and nut 322. The set screw 320 hasa tip 324 which is accommodated by a recess 326 in the outer wall of themounting head 308. The travel of the screw tip 322 within the recess 326defines the limit of travel of the sleeve 318 with respect to the head308. An annular collar or dam 328 is attached to the bottom of thesleeve 318 by means of a plurality of screws or bolts 330. The collar ordam 320 is configured so that a horizontal annular gap 332 is formedbetween the collar and the electrode mounting flange 314, which gapcommunicates with a vertical annular channel 334 formed between thecollar 328 and the electrode 312.

An electrolytic solution outlet 336 is provided in the mounting head 308and communicates through an outlet passage 338 with a cavity or chamber340 formed in the electrode 312. The juncture between the cavity orchamber 340 and the passage 338 is advantageously sealed by means of ano-ring 342.

A pair of electrolytic solution inlet passages 344 are provided in thehead 308. These passages 344 communicate with the annular gap 332through a pair of ports 346 in the electrode flange 314. The upper endsof the passages 344 communicate with an inlet manifold 348 in thesupport 302 through a second pair of ports 350 in the support 302. Thejunctures between the passages 344 and the flange ports 346 are sealedby o-rings 352, and the junctures between the passages 344 and thesupport ports 350 are sealed by similar o-rings 354.

A disc-shaped blank 360 illustrated in FIG. 24 and indicated by dashedlines in FIG. 19 is held in place by a holding plate 362 which in turnis aligned by a pair of rotating pins 364 over the locating base 306.The holding plate 362 has a top surface 366 which is abutted by aperipheral rim 368 around the electrode 312 when the support 302 ismoved down toward the base 306 to reach the bottom extent of its travelalong the alignment rods 304 and 304a.

Referring now to FIGS. 20-23, the proximal side forming electrode 312 isshown in detail. As best seen in FIGS. 20, 22, and 23, extendingdownwardly from the rim 368 of the electrode 312 is a proximal sideforming die face 370. The proximal side forming die face 370 has abottom surface 372 which has a trough 374 for forming the proximal sideof the strut 28a. As shown, the trough 374 conforms to the shape of theproximal side of the strut 28a and is sized somewhat larger than thestrut so that as the strut is being formed, a clearance space of between0.010 and 0.020 inches is maintained. A similar clearance space ismaintained between the side of the die face 370 and the valve ring 22aas the latter member is being formed. Running through the die face 370between the bottom surface 372 and the chamber or cavity 340 in theelectrode 312 are a plurality of electrolytic fluid channels 376. Thesechannels 376, which are advantageously located on both sides of thetrough 374, provide for a complete circulation of the electrolytic fluidas will be presently described.

The formation of the proximal side of the valve structure proceeds asfollows: A disc-shaped blank or workpiece 360 (FIG. 24) preferably of ametal such as Haynes-25 or a hard, corrosion resistant cobalt alloy, isretained in the holding plate 362. The height and diameter of theworkpiece or blank 360 are slightly greater than the height and diameterrespectively of the outside of the ring 22a of the heart valveprosthesis shown in FIGS. 6-11. The set screw 320 and nut 322 areloosened so that the sleeve 318 and collar 328 can be lowered until thebottom of the collar 328 comes into contact with the top surface 366 ofthe holding plate 362 and the set screw is then retightened to maintainthe sleeve and collar in this position. An effective seal is therebymaintained around the electrode 312 between the collar 328 and theholding plate 362. However, the set screw 320 is not set so tightly asto prevent the vertical travel of the mounting head 308 within thesleeve 318.

The support 302 is lowered along the alignment rods 304 until the bottomsurface 372 of the electrode 312 is brought to within approximately0.010 inches from the workpiece 360. When the apparatus is sopositioned, an electrolytic fluid (preferably a solution ofapproximately 1.25 pounds of sodium chloride per gallon of water) ispumped into the inlet manifold 348 at a pressure of approximately 175pounds per square inch. The electrolytic fluid then descends through thepassages 344, the annular gap 332, and the annular channel 334 into thenarrow gap formed between the workpiece 360 and the bottom surface 372of the electrode 312. The fluid fills the trough 374 and also flowsupwardly through the channels 376 into the chamber 340 in the electrode,from which it flows upwardly through the outlet passage 338 and out ofthe outlet port 336 for recirculation by filtering and pumping means(not shown) back to the inlet manifold 348. A back pressure ofapproximately 40 pounds per square inch is maintained at the outlet port336, thus providing a pressure differential of approximately 135 poundsper square inch between the inlet ports 350 and the outlet port 336. Inthis manner a constant high pressure, high velocity circulation ofelectrolytic fluid is maintained between the electrode and workpiece.Temperature regulating means (not shown) are incorporated into theelectrolytic fluid circulation system to maintain the temperature of thefluid at approximately 105° F.

While the electrolytic fluid is circulating as described above, apositive electrical potential is applied to the workpiece 360 and anegative electrical potential is applied to the electrode 312, with thedifference in potential between the electrode and workpiece beingapproximately 12 volts. This produces a current density of approximately1000 amperes per square inch of machined area to pass between theelectrode and the workpiece. This current flow causes metallic ions tobe electrically removed from the proximal surface of the blank orworkpiece 360 adjacent the bottom surface 372 of the electrode 312, andto pass into the electrolytic solution to be washed away through theflow of the fluid. This removal of metallic ions will result in anerosion of the workpiece, and in order to maintain the 0.010 inch gapbetween the workpiece and the electrode, the electrode must be loweredat a rate of approximately 0.110 inches per minute. As more and moremetal is removed from the workpiece and as the electrode is graduallylowered, the proximal side of the strut 28a will take a shape conformingto the trough 374 as shown in FIG. 22. In a similar manner, the innersurface of the proximal side of the ring 22a takes a shape approximatelyconforming to the walls of the die face 270. As shown in FIGS. 25 and26, the metal is almost completely eaten away by this process in theareas between the strut and the ring leaving a very thin layer of metal(approximately 0.0005 inches thick) in the area 378 and 380 which willbe the major and minor orifices, respectively, of the finished valve.

When the formation of the proximal side of the valve structure isfinished, the workpiece will have the form designated by the numeral360a in FIG. 25 and 26. The proximal sides of the valve ring 22a and thestrut 28a will be completely formed, nd the major and minor orifices 378and 380, respectively will be partially formed. At this point in theprocess, the mounting head has been lowered to the point where the rim368 of the electrode 312 has come into contact with the top surface 366of the holding plate.

Preparation must now be made for the formation of the distal side of thevalve structure. The flow of electrolytic fluid is shut off and theelectric potentials are removed from the electrode and the workpiece.The mounting head 308 is raised along the alignment rods 304, carryingwith it the electrode 312, the sleeve 318 and the collar 328. The collar328 is removed by removing the screws or bolts 330 which, as previouslymentioned, retain the collar on the sleeve 318. The proximal sideforming electrode 312 is then removed by removing the bolts 316 shown inFIG. 19A. It is replaced with a distal side forming electrode 412,illustrated in FIGS. 27, 28 and 29. The collar 328 is then reattached tothe sleeve 318. The thin metal layers in the areas 378 and 380 of theworkpiece are manually removed with a sharp instrument, and theworkpiece 360a is then flipped over and placed in the holding plate 362so as to present the distal side (FIG. 26) in a confronting relationshipwith the electrode 412. The collar or dam 328 is then lowered intoposition against the top surface 366 of the holding plate 362 aspreviously described, and the formation of the distal side of the valvestructure is ready to proceed in the same manner as in the formation ofthe proximal side.

Referring now to FIGS. 27, 28 and 29, the distal side forming electrodeor die 412 is seen to resemble the proximal side forming electrode 312in all respects except for the portion of the electrode in the area ofthe die face 470. Thus, when the distal side forming electrode 412 ismounted on the mounting head 308, the view taken along line A--A of FIG.19 would be identical to that shown in FIG. 19A.

The die face 470 of the distal side forming electrode 412 is segregatedfrom the body of the electrode by a peripheral rim 468 and has a bottomsurface 472 having a trough 474 for forming the distal side of the strut28a. Like the proximal side forming electrode 312, the distal sideforming electrode 412 is provided with electrolytic fluid channels 476communicating between the bottom surface 472 and an interior chamber ofcavity 440. These channels 476 serve the same function as the channels376 in the proximal side forming electrode 312.

As can be seen in FIG. 29, the trough 474 has a configurationcorresponding to the shape of the distal side of the strut 28a, but issized somewhat larger, for the purposes previously discussed. Thus, asthe blank 360a is eroded by the electro-chemical machining process, thedistal side of the strut 28a takes a shape conforming to that of thetrough 474 while the distal side of the ring 22a takes a shapeconforming to the walls of the die face 470. After the die has reachedthe bottom most point of its travel as previously described, theintegral strut and ring structure takes on its final configuration, withthe major and minor orifices 378 and 380, respectively, of the valvebeing completely formed between the strut and the ring. The final valvestructure designated by the numeral 500 is illustrated in FIG. 30. Ascan be seen from FIG. 30, the strut and the ring are formed by thismethod as an integral unit, and if the troughs 374 and 474 areappropriately configured, the strut 28a will have the modified teardropshape previously discussed. The structure 500 is now ready for a finalmachining by conventional means to provide the accurate sizing of thering and to provide the circumferential groove shown in FIGS. 6, 7 and11.

Although the advantages of electromechanically machining the openingsupport strut 26a are less pronounced, this can be accomplished, ifdesired, by providing an additional trough of appropriate configurationin each of the die faces 370 and 470.

Those of ordinary skill in the art of electochemical machining willrecognize that to accommodate variations in the characteristics in metalof which the disc-shaped blanks 360 are formed, it may be necessary toadjust the operational parameters of the electrochemical machiningprocess. Thus, in order consistently to achieve a satisfactory finishfor the valve structure 500, it may be necessary to adjust the rate atwhich the electrochemical machining process takes place. This may beaccomplished, for example, by varying the pressure differential in theelectrolytic fluid circulation system so as to increase or decrease therate of flow of electrolytic fluid across the workpiece. Alternatively,the electrical potential between the workpiece and the electrode may bevaried to adjust the current density between the workpiece and theelectrode, so as to maintain critical dimensions. Thus, the values forthese various parameters which are given above in the description of theprocess, while having produced totally satisfactory results, are merelyexemplary, in that it is within the abilities of those persons ofordinary skill in the art of electromechanical machining to find othervalues for these operational parameters which will yield satisfactoryresults for different conditions, requirements, and metalcharacteristics.

While the method and apparatus for producing the integral strut/ringstructure have been described only in conjunction with the heart valvewith arcuate occluder, (the embodiment of FIGS. 6-11), the use of thismethod and apparatus to formulate the integral ring/strut structures ofthe other prosthetic heart valve embodiments is disclosed above can beaccomplished in an almost identical manner, with the only major changebeing in the configuration of the die faces of the electrodes. With theteachings of this disclosure, it would be within the abilities of thoseof ordinary skill in the art of electrochemical machining to provideappropriately configured electrodes for the formulation of valvestructures for all of the valve embodiments disclosed herein, as well asother forms of heart valve prostheses which could make advantageous useof an integral strut/ring structure.

What is claimed is:
 1. A method for making an integral ring and strutstructure for a prosthetic heart valve having an occluder movablymounted in a ring by a strut, said method comprising the steps of:(1)placing a first side of a disc-shaped blank of hard, corrosion-resistantmetal into close proximity with a first electrode having a first dieface, said first die face being circular to form a ring of insidediameter approximately equal to the diameter of said first die face, andhaving a concavity for a strut which is integral with said ring; (2)flowing electrolytic fluid between said first die face and said blankwhile simultaneously applying an electric potential between saidelectrode and said blank to cause an electric current to flowtherebetween, said current causing metallic ions to be carried from saidblank into said fluid thereby eroding said blank in conformity with theconfiguration of said first die face; (3) continuing the flow of saidfluid and the application of said electric potential until said blank iseroded to a shape corresponding to the configuration of a first side ofsaid integral ring and strut structure; (4) placing a second side ofsaid blank into close proximity with a second electrode having a seconddie face, said second die face being circular to form a ring ofapproximately equal diameter to that ring formed by said first die face,and having a concavity for a strut which is integral with said ring andis matched to the concavity in said first die face; (5) flowingelectrolytic fluid between said second die face and said blank whilesimultaneously applying an electric potential between said electrode andsaid blank to cause an electric current to flow therebetween, saidcurrent causing metallic ions to be carried from said blank into saidfluid, thereby eroding said blank in conformity with the configurationof said second die face; and (6) continuing the flow of said fluid andthe application of said electric potential until said blank is eroded toa shape corresponding to the final configuration of said integral ringand strut structure.
 2. An integral ring and strut structure for aprosthetic heart valve having an occluder moveably mounted in said ringby said strut, wherein said integral ring and strut structure is made bythe method of claim
 1. 3. The integral ring and strut structure of claim2, wherein at least a portion of said strut has a cross-sectional shapesubstantially resembling a tear-drop.
 4. A method for making an integralring and strut structure for a prosthetic heart valve having an occludermoveably mounted in a ring by a strut, said method comprising the stepsof:(1) electrochemically machining one side of a disc-shaped blank ofhard, corrosion resistant metal using a first electrode having theconfiguration of a first die, said first die being circular to form aring, and having a concavity to form a strut which is integral to saidring; and (2) electrochemically machining the other side of said blankusing a second electrode having the configuration of a second die, saidsecond die being circular to form a ring of approximately equal diameterto the ring formed by said first die, said second die having a concavityfor a strut which is integral to said ring, and is matched to theconcavity in said first die.
 5. The integral ring and strut structuremade by the method of claim
 4. 6. The integral ring and strut structureof claim 5, wherein at least a portion of said strut has a non-circularcross-sectional shape which provides a low fluid dynamic flow resistancein proportion to cross-sectional area.
 7. The integral ring and strutstructure of claim 6, wherein at least a portion of said strut has across-sectional shape substantially resembling a tear-drop.
 8. Theintegral ring and strut structure of claim 5, wherein said ring andstrut structure comprises:a ring; and strut means for pivotally mountinga discoid occluder in said ring, said strut means being integral withsaid ring and extending across the interior thereof to provide a pivotfor said occluder.
 9. The integral ring and strut structure of claim 8,wherein said ring and said strut means each have an edge surface, saidring edge surface and said strut means edge surface being substantiallyflush with each other.
 10. The integral ring and strut structure ofclaim 5, wherein said ring and strut structure comprises:a ring having aproximal side and a distal side; and strut means for moveably mounting adiscoid occluder on the distal side of said ring, said strut means beingintegral with said ring and including a pair of parallel cage strutsextending distally from and across said distal side of said ring toprovide a stop for the movement of said occluder.
 11. The integral ringand strut structure of claim 5, wherein said ring and strut structurecomprises:a ring having a proximal side and a distal side; and strutmeans for moveably mounting a centrally apertured occluder on the distalside of said ring, said strut means being integral with said ring andcomprising:a cantilevered leg portion extending radially from said ringinwardly into the interior thereof and toward said proximal side, andterminating in an arcuate bend toward said distal side; and a postportion extending axially through the interior of said ring from saidbend to said distal side, said post portion providing means forconfining the movement of said occluder by slidably engaging saidoccluder through said central aperture.